As more successful therapy for certain cancers has evolved, secondary malignancy has become an increasingly important concern. In addition, recent progress in supportive care for patients undergoing high dose chemotherapy, with hematopoietic growth factors or hematopoietic system cell support, has led to a dramatic increase in the number of patients treated with prolonged and high dose DNA-damaging therapy. A method for studying the molecular effects of such therapy has been designed using cells made resistant to programmed cell death by overexpression of the apoptotic inhibitor Bcl-xL. Differential mRNA display has been used to compare gene expression in the presence and absence of chemotherapeutic agents with various mechanisms of action. This technique has resulted in the identification of a number of transcripts specifically upregulated by prolonged exposure to DNA-damaging chemotherapy. Among the transcripts identified are members of a family of retrotransposable elements. The role of mobile elements and of other transcripts identified by this technique in the cellular response to DNA damage will be explored. Programmed cell death is a critical defense mechanism against the continued proliferation of cells suffering major DNA damage or chromosomal aberrancy. Major genomic rearrangements and aneuploidy are frequent events in cancer, suggesting that the signaling pathway from DNA damage to programmed cell death is defective in the majority of cancer cells. Defects in this response pathway may be critical in permitting the genomic alterations involved in carcinogenic transformation. A genetic screen for factors involved in this apoptotic signaling pathway is being performed using a retroviral cDNA library in mammalian cells exposed to DNA-damaging agents. Exposure to DNA-damaging chemotherapy is associated with induction of programmed cell death, or apoptosis, in chemosensitive tumors. Exposure to these agents has also been associated with characteristic chromosomal rearrangements of secondary malignancies. This proposal seeks to better define both of these molecular pathways, from DNA damage to programmed cell death, and from DNA damage to chromosomal aberrancy. Understanding the molecular basis of the various cellular responses to DNA damage is relevant to the design of both chemopreventative and therapeutic anticancer strategies.